Chemistry Reference
In-Depth Information
geometrical structure of the molecules. Such dissociation processes are important
because of their scientific interest in chemistry, physics, and radiation biology, as
well as the possible applications in synthesizing new materials and chemical vapor
deposition in fabricating semiconductor devices. The purpose of the investigations
of the inner-shell excitation is to determine what happens to molecules following
the excitation and ionization of an inner-shell electron by using various kinds of
spectroscopic techniques, to define the initial photoexcitation process itself, and
to characterize and correlate the electrons, ions, neutrals, and metastable species
that are produced as a result.
The introduction of synchrotron radiation in the middle 1960s [10] gave the
real impetus to the whole field of atomic and molecular inner-shell spectroscopy.
Synchrotron radiation has a great advantage on its tunability over a wide spectral
range, extending from a few to several thousands of electron volts, compared to
other presently available sources [11, 12]. Synchrotron radiation combined with
suitable monochromators is a powerful research tool for the systematic investiga-
tions of outer- and inner-shell excitation and ionization processes in molecules,
because the spectral range matches the binding energy of valence and core electrons
related to the common elements forming molecules of physical and chemical in-
terest, namely, low-Z molecules. In addition to its tunability, synchrotron radiation
has many unique properties, such as polarization and pulsed time structure.
Although the pioneering molecular K-shell work on nitrogen (N
2
) in 1969 by
Nakamura et al. [13] with the use of synchrotron radiation has brought a certain
revolution in inner-shell spectroscopy of low-Z molecules, detailed understanding
of the inner-shell excitation spectra, has mainly been due to electron energy loss
spectroscopy (EELS) [14]. The main reason EELS had been predominantly used
for the investigation of molecular inner-shell excitation [15, 16], lay in the short-
age of resolution and intensity of the monochromatized synchrotron radiation until
the late 1980s, when high-performance monochromators became available. This
spectroscopy is a powerful and convenient tool to study the inner-shell excitation
with moderate resolution. However, this spectroscopy with high resolution is not
feasible above the oxygen
K
-edge excitation region because the large decrease in
signal intensity is unavoidable at higher energy losses. In addition, in order to ob-
tain the information on the relaxation process of a certain inner-shell excited state
using the electron impact method, it is indispensable to detect the products in coin-
cidence with inelastically scattered electrons. On the other hand, photon-induced
excitation has no special problem with the signal count rate at higher photon ener-
gies. During the 1990s, the experimental techniques associated with synchrotron
radiation have really made rapid progress together with the introduction of new
concepts on the design of monochromators [17-19] and the utilization of insertion
devices, especially since the so-called third generation soft X-ray synchrotron fa-
cilities (e.g., ELETTRA in Italy, ALS in the United States, MAX-II in Sweden,
and BESSY-II in Germany) became utilizable. Upgrade of the monochromatized
